1. Field of the Invention
The subject matter disclosed generally relates to structures and methods for fabricating solid state image sensors.
2. Background Information
Photographic equipment such as digital cameras and digital camcorders may contain electronic image sensors that capture light for processing into still or video images. Electronic image sensors typically contain millions of photoelectric conversion units such as photodiodes.
Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, photoelectric conversion units are formed in a substrate and arranged in a two-dimensional array. Image sensors typically contain millions of pixels each comprising a photoelectric conversion unit to provide a high-resolution image. To improve an efficiency of light capturing, certain image sensors have light-guides (or waveguides) to direct light towards the photoelectric conversion units. The light-guides may comprise a light transmissive material, for example silicon nitride such as Si3N4, having a refractive index higher than that of a surrounding insulating material, for example silicon oxide, so that there is a total internal reflection at sidewalls of the light-guides to keep light from exiting. Alternatively, the light-guides may have a metal coating on sidewalls to provide the reflection and are filled with a transparent material, for example silicon oxide or an organic resin or spin-on-glass (SOG). A pixel may comprise more than one light guides, one stacked above another, to form a cascaded light guide. The light guides at any one height from the substrate are typically at a given pitch from one another, and share a common horizontal cross-sectional profile at any one given height from the substrate. Keeping a constant pitch provides a uniform sampling of the image projected on the face of the image sensor along left-to-right and top-to-bottom directions (parallel to the plane of the pixel array), thus better matched to how pixels are arrayed on displays such as computer displays and prints.